Gas turbine engines have typically utilized labyrinth seals to minimize leakage fluid flow between rotating and non-rotating members. Labyrinth seals are also utilized to provide barriers against counterflows and thus intermixing of different fluids, such as high temperature working gas and lubricating oil, through gaps between rotating and non-rotating engine members. Labyrinth seals function to throttle fluid flow past a succession of annular constrictions provided between a series of circumferential teeth formed on a engine member rotating in closely spaced relation to a smooth mating surface carried by a stationary engine member. Seals of the labyrinth type have the advantages of simplicity and long operating life, and are reasonably effective fluid seals as long as the pressure differential across the seal is reasonably low.
Current advanced gas turbine engine designs to increase thrust and fuel efficiency subject fluid seals to a far more hostile environment in terms of greater pressure differentials, higher temperatures, larger seal diameters and increased velocity of the rotating engine member. Seal clearances must then be increased to account for manufacturing tolerances, possible eccentricity of the members, and material growth due to temperature and centrifugal loading. Excessive fluid leakage then occurs in a labyrinth seal to the detriment of engine performance and fuel economy. If the seal's function is to restrict leakage of high temperature working gas into engine bearing compartments or sumps, any lubricating oil in or near the seal can produce coke build-up which further degrades seal effectiveness.
To avoid the deficiencies of labyrinth seals encountered in advanced engine designs, gas bearing face seals are being substituted. An example of this type of fluid seal is disclosed in commonly assigned Moore U.S. Pat. No. 3,383,033. In a fluid seal of this type, a bearing in the form of a gas film is developed between the faces of a rotating mating ring and a non-rotating sealing ring to actively control the dimension of the sealing gap. The sealing ring is mounted by a carrier which, in turn, is mounted to a stationary housing for limited gap-varying movement relative to the mating ring face. To restrict fluid leakage between the carrier and the housing, a secondary fluid seal must be utilized. While significantly more effective in reducing fluid leakage than labyrinth seals, gas bearing face seals must be manufactured to exacting tolerances to ensure precise parallelism between mating and sealing ring face surfaces over wide ranges of temperature, pressure and speed. Also, measures must be taken to ensure a continuing force moment balance on the secondary seal to prevent unseating and consequent leakage. Gas bearing face seals are more effective in preventing leakage than labyrinth seals, since the gas film bearing can be extremely thin, e.g., less than 0.5 mils. However, with such close spacing between the mating and sealing ring face surfaces, intermittent contact therebetween can occur at low speeds, high pressure drops, or increased temperatures. Such contact results in wearing of the face surfaces with consequent reduced sealing effectiveness and service life.